1. Field of the Invention
This invention relates to frequency modulation devices in general, and in particular to a new and improved demodulation circuit for use with such devices. This invention is particularly useful in magnetic recording devices. This invention also relates to a new and improved method for demodulating frequency modulated signal.
2. Prior Art
In general, there are several types of demodulation circuits in use today. The direct or pulse-counter demodulator is most often used in magnetic recording devices and in particular in video tape recorders because of its linearity over such a wide deviation.
In pulse-counter demodulators the input signal, which typically comprises an intelligence signal in the form of a spectrum of frequencies modulated on a carrier frequency is limited to produce a square-wave signal having the same or proportional frequencies as the input modulated carrier signal. This square-wave signal is used to generate pulses of a fixed pulse width T. These pulses are next phase split to produce pulses at a rate proportional to the original carrier frequency. The phase splitting step is performed either before or after the limiting step. An average value of the pulses is next extracted by means of a low-pass filter to derive the intelligence frequency spectrum from the carrier frequency spectrum.
There are various means available for generating the pulses of fixed pulse width T from the square-wave signal. One popular means is by the use of a one-shot circuit typically comprising a multivibrator having a delay line coupled between an output terminal and a clear input terminal thereof. Thus, the pulse width T is determined by the amount of time required for a voltage transition to pass through the delay line added to other propagation time delays of the constituent circuitry.
There is a problem, however, with the above-described circuit in that at high frequencies the delay line will cause the one-shot circuit to eliminate or ignore pulses which occur during the time the multivibrator is switched off. When the input signal reaches a frequency having a period of less than twice the pulse width T, transitions or pulse derived from the input signal are lost since the delayed output of the multivibrator holds the multivibrator in a clear or off state.
Avoidance of this problem has been attempted by making the pulse width T more narrow by decreasing the time delay so as to accommodate higher input frequencies. However, a reduction in the pulse width T is limited by two factors. First, the sensitivity of the demodulator is reduced by a narrow pulse width T because of lower energy per pulse, which reduces the signal-to-noise ratio of the system. Second, the pulse width T is determined by a combination of the time delay produced by a delay line and by propagation delays of the constituent components of the demodulator circuits. These components are typically temperature-sensitive, which means that the propagation time delays will vary somewhat with temperature. Accordingly, if the time delay of the delay line is shortened such that the propagation delay time is any appreciable percentage of the total time delay, then the pulse width T becomes a variable of temperature and thus becomes inaccurate. Such timing variations will result in an inaccurate demodulation of a modulated frequency spectrum signal.